Metal-clad boards, particularly such boards for use in fabricating printed circuits, are well known in the art. Such metal clad boards generally comprise a resinous plastic substrate to which is bonded at least one thin sheet of an electrically conductive material. Particularly, the electrically conductive material is a metal foil formed of such materials as copper, aluminum, stainless steel and the like. The resinous plastic substrate can be clad with metal foil on one or both sides, depending upon its desired use. Metal-clad boards may be either rigid or flexible depending upon the composition of the resinous plastic substrate and the choice of the substrate is dependent upon the use to which the board is to be put.
In preparing rigid metal-clad boards, a previously prepared, thermoset plastic base, such as one formed of epoxy or modified polystyrene may be used. Alternately, the thermoset plastic base may comprise a number of plies of a reinforcement, such as paper or glass cloth, impregnated with a resin in the B-stage, commonly called pre-pregs. In either case, a metallic foil, such as copper, treated on one side to adhere to the thermoset plastic base, and sometimes coated with a layer of B-stage adhesive, is placed on top of the thermoset plastic base in a high pressure press, and the composite heated under pressure to effect a bond between the metal foil and plastic substrate. When a separate B-stage adhesive is not used, the resin of the resinous substrate adjacent the metal foil, acts as the adhesive. In the case of the pre-preg, the procedure also effects a cure of the substrate in the same process. If the copper foil has been subjected to a surface treatment involving a micro-roughening and controlled oxidation of the surface, an industry-wide practice developed by me, a separate adhesive layer may frequently be omitted. In such a case, the top layer of resin in the base acts as the bonding adhesive to the copper foil.
The curing cycle in the press will depend on the nature and thickness of the laminate, the time and temperature of the cycle being those required to cure the substrate and the bonding adhesive layer, if present. Sufficient pressure is required to effect adequate flow of the adhesive and/or substrate resins, in order to wet out and bond adequately. The pressure must also be sufficient to prevent blistering which is due to the release of gases resulting either from retained volatiles in the substrate or adhesive layers or resulting from byproducts of the curing process. Such byproducts may include water, carbon dioxide, and the like.
In press-curing, especially with large presses and with multiple openings, the pressure is not uniform over the entire area of the laminate due to platen deflection, and the like, and the cured laminate in turn is not homogenous in its properties. In an autoclave, on the other hand, the pressure is uniform throughout the chamber on all areas of the laminate in all directions.
When forming metal-resin laminates in long lengths, the high pressure press cannot be employed. Prior techniques for curing long lengths of flexible metal-clad resinous plastic substrates have included merely passing the resinous plastic lamina and metal foil through a nip roll, or carrying the resinous plastic substrate metal foil laminate which has been previously nip rolled through a long oven so that heat was applied for a considerable period of time. Neither of these techniques has proven effective. In the case of nip roll bonding alone, the bond is inferior, particularly at elevated temperatures such as is experienced in subsequent soldering operations, so that there is considerable solder blistering, i.e., blistering between the metal foil and plastic substrate, when the laminate is soldered. In passing the laminate through a long oven, the foil is generally imperfectly bonded to the resinous substrate because of the expansion and escape of residual volatiles in the resin, either the resin in the substrate or in the adhesive and, in addition, the metal-clad substrate is subject to severe mechanical damage, including dents, scratches and the like. Processing in a long oven of this type is, further, uneconomical because of the time that the laminate must be subjected to heat in the oven.